The present application claims priority upon Japanese Patent Application No. 2001-223955 filed on Jul. 25, 2001, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates generally to a beam shutter for providing a control of transmission and interruption of output light beams, and more particularly to a magneto-optical beam shutter having a plurality of plane-parallel birefringent elements and a variable Faraday rotator, configured to provide an on/off control of light beams by changing the direction of polarization by means of the variable Faraday rotator.
2. Description of the Related Arts
In the field of optical communications, optical measurement systems, etc., a beam shutter is incorporated as a protection feature for a light receiving element. When a main switch is turned on for example, high energy light beams may occur instantaneously (e.g., during several milliseconds to several seconds) in association with the systems, which may possibly break down the light receiving element. Thus, the beam shutter is disposed at the input side of the light receiving element so that control is provided to activate an optical path after light beams reach a certain stable level. Otherwise, any causes may possibly result in an instantaneous occurrence of the high energy light beams.
The conventional beam shutter is a mechanical one. For instance, arrangement is such that a mirror is disposed on the optical path and driven by a motor. The orientation and location of the mirror are changed so as to control the transmission and interruption of the output light beams.
However, the mechanical beam shutter drives the mirror and other optical components by means of a drive mechanism such as the motor, which makes the apparatus larger and susceptible to degradation in characteristics due to wear or damage of the movable parts, impairing the reliability. A longer time is required for the drive thereof and instant actions cannot be dealt with.
It is therefore one object of the present invention to provide a magneto-optical beam shutter capable of a rapid switching between the transmission and interruption due to having no movable parts, and capable of securely protecting the light receiving element and the like.
In order to attain the above and other objects, according to a first aspect of the present invention there is provided a beam shutter comprising a plane-parallel birefringent element for separation arranged to separate input light beams having orthogonal polarization directions on the same optical path; a plane-parallel birefringent element for synthesis arranged to synthesize light beams having orthogonal polarization directions on different optical paths, for output; the birefringent element for separation and the birefringent element for synthesis being disposed in a spaced apart relationship; and polarization rotating means having a variable Faraday rotator, the polarization rotating means interposed between the birefringent element for separation and the birefringent element for synthesis, for controlling the polarization directions; wherein transmission and interruption of output light beams are controlled by switching the polarization directions by means of the variable Faraday rotator.
In this case, the polarization rotating means may include a combination of a xc2x145 degree variable Faraday rotator and a xc2xd wave plate having an optical axis defined to rotate the polarization directions through 45 degrees. Alternatively, the polarization rotating means may include, in the mentioned order, a xc2xd wave plate arranged to rotate the polarization directions through 45 degrees, a pair of polarizers having optical axes symmetrically defined on both optical paths so as to allow light beams transmitted through the xc2xd wave plate to intactly pass therethrough, and a xc2x145 degree variable Faraday rotator.
According to a second aspect of the present invention there is provided a beam shutter comprising a plane-parallel birefringent element for separation arranged to separate input light beams having orthogonal polarization directions on the same optical path;
a plane-parallel birefringent element for optical path control arranged to control optical paths depending on the polarization directions; a plane-parallel birefringent element for synthesis arranged to synthesize light beams having orthogonal polarization directions on different optical paths, for output; the birefringent element for separation, the birefringent element for optical path control and the birefringent element for synthesis being disposed in a spaced apart relationship; and polarization rotating means interposed between the birefringent element for separation and the birefringent element for optical path control and between the birefringent element for optical path control and the birefringent element for synthesis, the polarization rotating means rotating the polarization directions through 45 degrees, at least one of the polarization rotating means having a xc2x145 degree variable Faraday rotator; wherein transmission and interruption of output light beams are controlled by switching the polarization directions by means of the variable Faraday rotator.
According to a third aspect of the present invention there is provided a beam shutter comprising a plane-parallel birefringent element for separation arranged to separate input light beams having orthogonal polarization directions on the same optical path; a plane-parallel birefringent element for optical path control arranged to control optical paths depending on the polarization directions; a plane-parallel birefringent element for synthesis arranged to synthesize light beams having orthogonal polarization directions on different optical paths, for output; the birefringent element for separation, the birefringent element for optical path control and the birefringent element for synthesis being disposed in a spaced apart relationship; first polarization rotating means interposed between the birefringent element for separation and the birefringent element for optical path control, for converting the polarization directions from orthogonal to parallel; and second polarization rotating means interposed between the birefringent element for optical path control and the birefringent element for synthesis, for converting the polarization directions from orthogonal to parallel; at least one of the first and second polarization rotating means having a xc2x145 degree variable Faraday rotator; wherein transmission and interruption of output light beams are controlled by switching the polarization directions by means of the variable Faraday rotator.
According to a fourth aspect of the present invention there is provided a beam shutter comprising a first plane-parallel birefringent element for separation arranged to separate input light beams having orthogonal polarization directions on the same optical path; a first plane-parallel birefringent element for synthesis arranged to synthesize light beams having orthogonal polarization directions on different optical paths, for output; the first birefringent element for separation and the first birefringent element for synthesis being disposed in a spaced apart relationship; a second plane-parallel birefringent element for separation arranged to separate input light beams having orthogonal polarization directions on the same optical path; a second plane-parallel birefringent element for synthesis arranged to synthesize light beams having orthogonal polarization directions on different optical paths, for output; the second birefringent element for separation and the second birefringent element for synthesis being disposed in a spaced apart relationship and located posterior to the first birefringent element for separation and the first birefringent element for synthesis; first polarization rotating means having a variable Faraday rotator, the first polarization rotating means interposed between the first birefringent element for separation and the first birefringent element for synthesis, for controlling the polarization directions; second polarization rotating means having a variable Faraday rotator, the second polarization rotating means interposed between the second birefringent element for separation and the second birefringent element for synthesis, for controlling the polarization directions; wherein transmission and interruption of output light beams are controlled by switching the polarization directions by means of the first and second variable Faraday rotators.
In the above aspects of the present invention, the variable Faraday rotator may comprise a Faraday element and variable magnetic field applying means, the variable magnetic field applying means being comprised of magnetic circuits each having a coil and a gap, the direction of Faraday rotation being changed through the control of the direction of magnetic field applied. In the event of incorporating a plurality of variable Faraday rotators, a corresponding number of magnetic circuits each having a coil may be provided for individual Faraday elements, but instead, a single (common) magnetic circuit having a coil and a plurality of gaps may also be used. The magnetic circuit may be made of a semi-hard magnetic material so that the direction of Faraday rotation can be kept even though current flowing through the coil is cut off, thereby making the power saving possible.